1. Field of the Invention
The present invention relates to a control apparatus for controlling an automatic transmission, and more particularly to an automatic transmission control apparatus that changes transmission gear ratios by use of a motor.
2. Description of the Related Art
There is publicly known an automatic transmission that changes the gear ratios of a transmission mechanism by operating a transmission lever through a shift direction motor and a selection direction motor. Each of the shift direction motor and the selection direction motor that are utilized in such an automatic transmission may be, for example, a three-phase AC motor that is driven by a DC power source by way of an inverter.
To date, among apparatuses that drive a three-phase AC motor through an inverter, there has been known a motor control apparatus (e.g., refer to Japanese Patent Application Laid-Open No. 2005-117756) that has a voltage detection unit for detecting a DC voltage as an input voltage of an inverter and a motor current detection unit for detecting a motor current, that drives a motor while selecting one of PWM (Pulse-Width Modulation) control and rectangular wave control, that detects abnormality in the voltage detection unit, based on a detected amount of change in the DC voltage or in the motor current, and that, in the case where it is determined that an abnormality exists in the voltage detection unit, fixes the control of the motor to the PWM control.
In the motor control apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-117756, in the case where the voltage detection unit fails, a DC voltage (input voltage of the inverter) outputted from the voltage detection unit is fixed to a minimum guaranteed voltage. The minimum guaranteed voltage is set in such a way as to be the minimum value among values of the input voltage of the inverter. The PWM control is performed as follows. Current feedback control is performed in accordance with a d-axis current command value based on a torque command and a current difference obtained from a d-axis current (motor current); a voltage command value is calculated based on the current feedback control; after a superposition voltage is added to the voltage command value, processing such as a coordinate transformation is performed so as to calculate three-phase voltage command values; the three-phase voltage command value is normalized with respect to the inverter input voltage, i.e., a duty ratio is calculated; and the inverter is PWM-controlled through a pulse width calculated based on the duty ratio so that a motor is operated.
However, in the conventional apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-117756, by fixing the input voltage of the inverter to the minimum value in the case where the voltage detection unit fails, the duty ratio is increased. For example, in the case of a normal state, the duty ratio is calculated with respect to a normal inverter input voltage (e.g., 12 [V]); however, in contrast, in the case where the voltage detection unit fails during a control state in which the voltage command value becomes constant, the duty ratio is calculated with respect to the minimum inverter input voltage (e.g., 8 [V]), whereby the duty ratio increases.
In this situation, in the case where the inverter input voltage, which cannot be detected, is the same as the fixed value, even though the duty ratio increases, the voltage utilized for calculating the duty ratio is the same as the actual voltage; therefore, a voltage equal to the voltage command value can be applied to the motor. However, in the case where the actual inverter voltage is higher than a fixed value such as a normal-state voltage, the motor is operated with an increased duty ratio; therefore, a voltage higher than the voltage command value is applied to the motor. Accordingly, there has been a problem that torque greater than a torque command value is produced and a current greater than a current command value flows, whereby a motor cannot safely be operated. Additionally, there has been a problem that, in the case where torque greater than a torque command value is produced, a motor is erroneously operated, whereby the controllability of the motor operation is deteriorated.